1. Field of the Invention
The invention relates to finger-print reading systems used especially in devices for authenticating individuals.
2. Description of the Prior Art
The many systems used to authenticate individuals, based on fingerprint analysis, comprise at least one sensor used to obtain an image of the fingerprint of the individual to be identified. In presently used systems, the finger is placed on the sensor whose reading surface must necessarily have a size equal to about the size of the finger. The sensor is associated with a system of analysis used to compare the image of the fingerprint that it gives with an image of a reference fingerprint stored in an appropriate medium, for example a chip card.
In most cases, the sensors give an analog type of information element and the analysis system makes use of an operation for the digital processing of the image of the fingerprint which must be digitized at output of the sensor by means of an analog-digital converter. In certain structures, the sensor delivers the digitized image directly.
Fingerprint-reading systems are often based on the use of optical devices such as a video camera picking up the image of the finger but it is possible, with a simple photograph of the same finger, to obtain the same image at output of the camera and thus defraud the system. To overcome this drawback, certain systems use prisms or microprisms in order to ascertain that it is really a genuine finger and not a photograph that is being placed before the sensor, the light being reflected only at the places where the lines of the fingerprint do not touch the prism. A photograph thus becomes inoperative. However, the optical systems cannot be used to ascertain that the finger which has been placed before the sensor is truly a live finger and is not for example a molded finger. The optical systems have other drawbacks such as for example their great volume and high cost of production.
Other means have been proposed to make devices for the authentication of individuals by means of fingerprints, making use of the batch-processing possibilities of the semiconductor industry. Devices made in this way are potentially less costly and offer the advantages of the integration of the sensor and of all or a part of the data-processing sequence of the authentication device, especially the operations of image digitizing at output of the sensor, the storage of the reference image and authentication. The fingerprint-reading sensor has a matrix of sensitive elements organized in rows and columns, giving an electric signal that differs depending on whether a ridge of the fingerprint line touches or does not touch a sensitive element of the sensor.
Patents have been filed on various means of reading fingerprints:
The U.S. Pat. No. 4,353,056 describes a principle of reading based on the variation of the capacitance of the sensitive elements of the sensor.
Other systems comprise sensors having components sensitive to pressure, temperature or else to pressure and temperature, converting the spatial information on pressure and/or temperature into an electric signal that is then collected by a semiconductor-based multiplexer which may for example be a CCD matrix. The U.S. Pat. No. 4,394,773 describes a principle of this kind.
The sensors based on the piezoelectric and/or yroelectric effects are the most valuable for they are sensitive to the pressure and/or to the heat exerted on their sensitive elements. This feature makes it possible to ascertain, during the reading of fingerprints, that the finger is truly part of a living individual, through the inherent heat that it releases. It is also possible to detect the variations due to the pulsating flow of blood in the finger, inducing a variation of heat and/or pressure, thus providing for greater reliability in the authentication of the fingerprint.
The electric signal given by the sensors integrated into a semiconductor substrate has the drawback of being short-lived and a specific system is necessary to maintain it in time. The signal is short-lived because the electric charges are induced by variations of the physical effects (temperature, pressure, etc.) on the sensor. As a consequence, the signal at its output tends to disappear when the physical effects are brought into equilibrium. The time constants of disappearance of the signal are in the range of some milliseconds to some seconds in the more favorable cases.
The practical result of this is that a series of images is produced starting from the time when the finger is placed on the sensor. The quality of contrast of these images is not stable, and they tend to fade away. This complicates the task of the recognition system for it is then necessary to analyze all the images that are being constantly produced by the sensor in order to find the one most suitable for authentication.
Systems with excitation external to the sensor have been proposed. These are, for example, systems for sending an energy beam in the form of microwaves. However, they complicate the system and increase its volume and cost.
It is possible to mitigate the effects of the disappearance of the image of the fingerprint by means of an electronic memory. However this complicates the designing of the sensor and increases its cost of manufacture for it requires technology that enables memory storage. It is very difficult to build a system that is sufficiently precise, reliable and inexpensive, and capable of deciding which is the best image among all those produced by the sensor.